1. Field of the Invention
The present invention relates to a device for locking the manual operation ring for exposure control on a photographing lens. More particularly, the present invention is directed to such locking device capable of locking the manual operation ring at a determined position for the change-over between manual control mode and automatic control mode.
2. Related Background Art
In a known photographing lens provided with a manual operation ring, the exposure control mode is changed over from manual mode to automatic mode when the manual operation ring is moved to a certain determined position. At this particular set position, a locking device is actuated to lock the manual operation ring against further rotation. In the art, there have been known various locking means for this purpose.
In a typical example of the photographing lens with an aperture-presetting ring, a locking device is provided on a stationary ring opposed to the aperture-presetting ring. In the automatic aperture control mode including shutter time priority automatic exposure control mode, programmed automatic exposure control mode etc., the operator presets the aperture-presetting ring in the position for minimum aperture value. The locking device locks the presetting ring against any accidental rotation of the presetting ring.
In a known construction, the locking device is so formed that it can be actuated automatically when the aperture-presetting ring has just been rotated to a predetermined lock position indicated by a mark. The lock position mark is at a point a little forward beyond the minimum aperture value position of the presetting ring. This known construction has some drawbacks. Firstly, a particular mark indicating the minimum aperture value position for aperture control mode is needed in addition to the mark indicating the lock position. Secondly, the aperture-presetting ring has to be moved over a larger rotational angle which is inconvenient to the user of the camera.
In another known photographing lens, the locking device is so formed that it can be actuated automatically at once when the aperture-presetting ring reaches the position for minimum aperture value. However, the use of the known locking means limits the freedom of aperture selection in other exposure control modes than the automatic aperture control mode. For example, in the manual exposure control mode in which the operator can manually set the aperture to any desired value, or in the aperture priority automatic exposure control mode, there may occur such case where the operator wishes to change the aperture from minimum aperture value to another value. But, the known locking device inhibits such a selection of aperture value because the aperture-presetting ring has already been locked in the position for the minimum aperture value in this case.
To overcome the drawback of the known locking means as mentioned above, a solution has been proposed, for example, by the invention of U.S. Pat. No. 4,341,450 (patented on July 27, 1982). According to the known solution, a slidable lock member with a projection is provided on any one of the stationary ring and the aperture-presetting ring. The lock member is mounted for manual slide-movement in the direction along the optical axis.
In a predetermined position on the other ring, there is formed a recess in which the above-mentioned projection is engageable. The aperture-presetting ring can be locked only when the projection gets in engagement with the recess by the operator's moving the lock member. The important disadvantage of this known solution is found in that the manual operation required for locking is complicate and takes a long time. The reason for this is that the operator has to rotate at first the preset ring up to the position in which the projection and the recess are opposed to each other, and then to move the lock member in the direction along the optical axis in order to establish the locking engagement.